welcome to tyco integrated security. with world-class monitoring centers and thousands of qualified technicians. we've got a personal passion to help your business run safer, smarter, and sharper. we are tyco integrated security. and we are sharper. >>> we just got a live update here in boston. investigators are still interviewing witnesses they say. they're trying to put together a time line and they're particularly asking for help from the public. and explosives specialists and forensic specialists will be brought to the scene. >> this remains a very active investigation. our ongoing investigation in various locations throughout the area goes on. however, there are no known additional threats. we continue to interview various witnesses and process the crime scene. >> i want to bring in michael lighter, nbc news terrorism expert and former director of the national counterterrorism center and nbc news justice correspondent pete williams. michael, let me ask you about the things we heard at the news conference. hundreds i